This invention relates generally to smectite clays, and more specifically relates to a process for treating such clays in order to facilitate exfoliation, a phenomenon which is particularly significant in the preparation of clay-polymer nanocomposites.
As utilized in the present specification, the term xe2x80x9csmectitexe2x80x9d or xe2x80x9csmectite-type claysxe2x80x9d refers to the general class of clay minerals with expanding crystal lattices, with the exception of vermiculite. This includes the dioctahedral smectites which consist of montmorillonite, beidellite, and nontronite, and to the trioctahedral smectites, which includes saponite, hectorite, and sauconite. Also encompassed are smectite-clays prepared synthetically, e.g. by hydrothermal processes as disclosed in U.S. Pat. Nos. 3,252,757; 3,586,468; 3,666,407; 3,671,190; 3,844,978; 3,844,979; 3,852,405; and 3,855,147.
For a number of years interest has been developing in processes which are useful in producing composite materials composed of a organic polymer and a smectite-type clay mineral, with the mineral being connected to the polymer through ionic or other bonding. For example, in Japan Laid Open Application S51(76)-109998 (owned by Unichika, K.K.) and entitled xe2x80x9cMethod for Manufacturing a Clay-Polyamide Compositexe2x80x9d, a method is disclosed for manufacturing a clay-polyamide composite characterized by carrying out the polymerization of lactam in the presence of a clay-organic compound composite made by carrying out ion exchange to bond an organic compound which contains at least one amino group and has the catalyst effect of polymerizing the lactam and clay. The organic compounds mentioned include omega-aminocapronic acid, a nylon salt, hexamethylenediamine, and aminodecanoic acid. The lactams include epsilon-caprolactam and others such as omega-enantolactam, omega-capryllactam, and omega-laurolactam. The clays used include the montmorillonite group of clay minerals such as montmorillonite, hectorite, etc; and other clays are listed. Montmorillonite is preferred because of the high exchange capacity. The composite is made by first ion exchanging the clay with the organic compound under aqueous conditions, after which the which the suspension is washed, filtered and dried, then crushed. (This is essentially the conventional procedure for preparing a conventional organophilic clay, i.e. an xe2x80x9corganoclayxe2x80x9d.) The xe2x80x9corganoclayxe2x80x9d and lactam are mixed, with the organoclay being 10 to 75 wt % of the mixture. During mixing the mixture is brought to 80-100 deg C. to melt the lactam. Polymerization is carried out at 240 to 260 deg C. In the resulting composite product it is stated that the silicate layer has a thickness of 9.6 Angstroms. In a first example the interlayer distance of the organoclay layers before polymerization was 3.4 Angstroms, and 13.1 Angstroms after polymerization. In Example 4 the interlayer distance was 6.5 Angstroms before polymerization and 50.6 Angstroms after polymerization. The composite produced in this publication is stated to have good fire-retardant properties, and improved mechanical properties. Related disclosures are found in U.S. Pat. Nos. 4,739,007; 4,810,734; and 4,889,885.
The phase dispersions exhibited by the composite materials thus far discussed are relatively coarse, and differ materially in this respect from nanocomposites. The latter are a relatively new class of materials which exhibit ultrafine phase dimensions, typically in the range 1-1000 nm. Experimental work on these materials has generally shown that virtually all types and classes of nanocomposites lead to new and improved properties when compared to their micro- and macrocomposite counterparts. The number of nanocomposites based on smectite-type clays and linear thermoplastics is growing. Wang and Pinnavaia, e.g., have recently reported delamination of an organically modified smectite in an epoxy resin by heating an onium ion exchanged form of montmorillonite with epoxy resin to temperatures of 200-300xc2x0 C. Chemistry of Materials, vol. 6, pages 468-474 (April, 1994). A further example appears in U.S. Pat. No. 5,554,670, where an epoxy-silicate nanocomposite is disclosed which is prepared by dispersing an organically modified smectite-type clay in an epoxy resin together with diglycidyl ether of bisphenol-A (DGEBA), and curing in the presence of either nadic methyl anhydride (NMA), and/or benzyldimethyl amine (BDMA), and/or boron trifluoride monoethylamine (BTFA) at 100-200xc2x0 C. Molecular dispersion of the layered silicate within the crosslinked epoxy matrix is obtained, with smectite layer spacings of 100 xc3x85 or more and good wetting of the silicate surface by the epoxy matrix. Additional recent references evidencing the increasing interest in nanocomposites incorporating organoclays in polymer matrices include U.S. Pat. Nos. 5,164,440; 5,385,776; 5,552,469; and 5,578,672.
In a typical procedure for preparing a nanocomposite, the smectite clay, most commonly a montmorillonite, is treated with an organic ammonium ion to intercalate the organic molecule between the silicate layers of the clay, thereby substantially swelling or expanding the interlayer spacing of the smectite. Thereafter the expanded silicate layers are separated or exfoliated in the presence of or with the assistance of a polymer with which reactive groups on the intercalated organic molecule are compatible. A monomer can also be used which is polymerized after being intermixed with the intercalated clay. High shear mixing may be used as part of the exfoliation process.
Now in accordance with the present invention, it has unexpectedly been discovered that edge treatment of smectite clays, such as montmorillonite, with negatively charged organic molecules increases the exfoliation efficiency achieved in preparing nanocomposites. While it has heretofore been known to subject smectite clays to surface treatment incident to preparing a nanocomposite; and while it is also known to edge treat a smectite clay with negatively charged polyanions where the treated clay is subsequently used as a reinforcing filler in rubber and the like (see U.S. Pat. No. 2,883,356); it has not to my knowledge been known to utilize such edge treatment to augment exfoliation of smectites in the preparation of nanocomposites.